It is currently unknown as to how the FBI hacked into the iPhone 5C, there are however speculation that a device called 'IP Box' which is widely believed to have accessed the San Bernardino iPhone also is speculated that it can access to the iPhone 6.

Apple's attorneys have mentioned that they don't know what method was used to unlock Syed Rizwan Farook's iPhone 5C, the companies attorney have states that "normal product development would eventually address whatever exploit was used."

Government officials said they are confident that the method used by the FBI would not be a security concern for most users, according to ZDNET. James Comey, director of the FBI revealed that the hack used won't unlock anything newer than the iPhone 5C. Apple has began implementing secure hardware which would make it more difficult to crack into a passcode locked iDevice.
"This doesn't work on 6S, doesn't work on a 5S, and so we have a tool that works on a narrow slice of phones," James Comey said.
James Comey also stated "We tell Apple, then they're going to fix it, then we're back where we started from," he said. "We may end up there, we just haven't decided yet."
There has been wide speculation regarding a device called "IP Box" which appeared last year. The 'IP Box' which retails for around $300 U.S. dollars latches on to the devices power circuitry system and enters PINs over a USB cable. When a incorrect guess is entered, the tool cuts power to the iPhone's logic board before the guess is recorded, which defeats the 10 time limit Apple implemented in iOS 8.
However, Apple is rumored to have patched the hole in the older iPhone's with iOS 8.1.1; the iPhone 5C which was used by Syed Farook is rumored to run iOS 9. So it is questioned whether the FBI has chosen an alternative method or has purchased the 'IP Box' which have discovered another flaw in later software versions of iOS.
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